Improvements in and mountings for radiation detecting devices

ABSTRACT

Improved gas discharge radiation detecting devices have long life spans under severe environmental conditions and under high vibration. A first, tubular electrode surrounds a second, elongated wire electrode with a hermetically sealed gas ionization chamber therebetween. A pair of electrode mounting means mount and tension the wire electrode through friction joints to provide sufficient tension to prevent electrical failure under severe vibration conditions. It has been found that electrical failure of such devices can be caused by vibration conditions which damage a protective surface oxide of the wire electrode and that proper mounting can eliminate such damage thereby greatly prolonging useful life.

United States Patent [191 Cocks et al.

[ Jan. 8, 1974 1 IMPROVEMENTS IN AND MOUNTINGS FOR RADIATION DETECTING DEVICES [75] lnventors: Franklin 11. Cocks, Waltham;

Herbert E. Bates, Sudbury, both of Mass.

[73] Assignee: 'lyco Laboratories Inc., Waltham,

Mass.

22 Filed: Sept. 29, 1971 [21] App]. No.: 184,704

[52] US. Cl 313/93, 313/216, 313/217, 313/218, 313/269, 313/278 [51] Int. Cl. H01j 39/30 [58] Field of Search 313/93, 269, 278, 313/218, 216, 217; 250/836 R [56] References Cited UNITED STATES PATENTS 2,552,723 5/1951 Koury 313/93 2,974,247 3/1961 Anton 313/93 Primary ExaminerPalmer C. Demeo Att0rney-Wolf, Greenfield & Sacks [57] ABSTRACT Improved gas discharge radiation detecting devices have long life spans under severe environmental conditions and under high vibration. A first, tubular electrode surrounds a second, elongated wire electrode with a hermetically sealed gas ionization chamber therebetween. A pair of electrode mounting means mount and tension the wire electrode through friction joints to provide sufficient tension to prevent electrical failure under severe vibration conditions. it has been found that electrical failure of such devices can be caused by vibration conditions which damage a protective surface oxide of the wire electrode and that proper mounting can eliminate such damage thereby greatly prolonging useful life.

21 Claims, 3 Drawing Figures 1 IMPROVEMENTS IN AND MOUNTINGS FOR RADIATION DETECTING DEVICES BACKGROUND OF THE INVENTION Gas discharge radiation detecting devices such as Geiger-Muller tubes or counters have long been known. Normally such tubes comprise a confined gas positioned between two electrodes; one of which is cylindrical and used as the cathode with the other a fine wire acting as an anode. The anode is normally stretched along the axis of the cylindrical cathode and has a protective surface oxide to preventchemical reaction with the gas atmosphere during high temperature operation. An electrical potential is placed on the wire anode and an inert gas mixed with a hologen is contained within a chamber between the anode and cathode. When a charged particle passes through the gas, ionization of atoms of the gas occurs with electrons produced drawn toward the central anode. It is believed that the electrons set free are then collected as a charge on the central wire or anode. As is known, pulses coming from the Geiger-Muller tube are amplified electronically and can then becounted by an electromechanical register or other counting means to determine and detect charged particles usually as a function of time.

In conventional Geiger-Muller counters, one end of the wire anode is mounted on a metal end cap as by a crimping operation forming a plurality of noninterconnected wedge-shaped indentations with the second end beingmounted by an enlarged bulbous end stretched against a ceramic end plug.

Conventional Geiger-Muller tubes as previously described often have relatively short useful lives under corqj isnsstua it htuiaa. Pe ennials, at Queuin temperatures of 135C and accelerations of 20 (PS, known Geiger-Muller tubes frequently last as little as 12 hours before erratic counting results are obtained and complete electrical breakdown occurs.

SUMMARY OF THE INVENTION It has now been found that a main cause of electrical breakdown of known Geiger-Muller tubes under severe vibration conditions, is the formation of small cracks in the anode oxidized surface which permit exposure of the underlying anode material to the gaseous atmosphere which in turn causes electrical destruction of the anode and resulting failure of the tubes.

It is a primary object of this invention to provide gas discharge radiation detecting devices which have long useful lives even under severe conditions of vibration.

It is a further object of this invention to provide devices in accordance with the preceding object which can be efficiently manufactured and which are highly accurate over long periods of use.

Still another object of this invention is to provide improved surface protection for anode wires in gas discharge devices to enhance useful life over a wide range of conditions of use.

Still another object of this invention is to provide novel mounting means for a wire anode in a gas discharge device to provide proper tensioning and positioning of the anode.

According to the invention a gas discharge radiation detection device in the form of a Geiger-Muller tube has a first tubular electrode and a second elongated electrode spaced from and within the first tubular electrode. Ceramic end cap means define in part a hermetically sealed gas ionization chamber between the first and second electrodes. A pair of second electrode mounting means mount the second electrode in tension. The mounting means are each mechanically joined to the second electrode by friction joints at axially spaced portions of the electrode. Preferably, the second electrode a wire is tensioned to at least 60FTO pounds per square inch and no more than 67 percent of its yield point. Preferably the wire electrode has high tensile strength and is formed of a material, such as A.I.S.I. stainless steel l7-7PH which has substantially similar thermal expansion characteristics as the tubular electrode such as normally used A.I.S.I. stainless steel 446.

The mounting means for the center electrode are joined to the central electrode by swaging over the entire circumference of the electrode without substantially reducing the diameter of the electrode thereby forming a friction joint. It is a feature of this invention that the joints provide a self-regulating characteristic to the center electrode. Thus, if the electrode is subjected to higher mechanical tension than desired, the electrode slips with respect to the friction joints rather than elongating or breaking thereby maintaining mechanical tension within a desired range. The tension load at which the wire slides through the joint is always less than the yield strength of the wire. The use of swaging, i.e., uniform reduction of the wire anode circumference to a predetermined degree, rather than crimping with a predetermined force, aids in obtaining uniform desired characteristics in resultant devices.

In a preferred embodiment, the wire electrode is coated with a thin coating of surface treated chromium to enhance its resistance to surface attack by a halogencontaining atmosphere normally used in the chamber between the two electrodes.

When a conventional gas discharge radiation device such as a Geiger-Muller tube having a cylindrical cathode and wire anode is subject to vibration in use, the anode wire vibrates, and if the vibration is of a frequency at or near the resonant frequency of the wire, its vibrational amplitude is large. This vibration of the anode wire placesa stress upon the end caps used to secure the anode along the axis of the cylinder. The stress acts in addition to the stress applied by the tension under which the wire is normally held. The stress is concentrated at the points where the anode wire is affixed to end cap supports. The resonant frequency of the center anode wire increases while its maximum displacement decreases as the tension is increased. These effects are both beneficial. In the case where the increase in resonant frequency is obtained, improvement is obtained because in many vibration situations, the maximum accelerations experienced by the tube decrease as-the frequency with which they are applied increases. Thus, a tube with a higher resonant frequency will, under these conditions, experience less centerwire displacement than one with a lower resonant frequency. A decrease in the displacement of the center wire anode is an advantage because both the flexing of this wire, as well as the neamess of its approach to the cathode are decreased. It is desirable that flexing be avoided because it tends to crack any protective surface oxide which is normally formed on the wire, and cracking allows underlying metal of the wire to react with halogen quenching gases normally present, thus leading to electrical failure of the tube as the halogen gas is eliminated from the gaseous atmosphere. Flexing of the center anode wire also leads to a variation in starting voltage because of the approach of the central anode'wire to the cathode tube. Thus, it is a feature of this invention that it has been found that high mechanical tensions are desirable in the central anode wire. High tensions are obtained by positive securing of each end of the anode wire by a friction joint as well as use of an anode wire of high strength which when oxidized at the surface, is compatible with the halogen quenching gas normally used. Thus, it is preferable to use high strength anode wire such as stainless steels containing a substantial amount of chromium. In addition to using a high strength wire, particular securing means for forming the joint between the end caps and the wire are used to enable retention of high mechanical tension in the anode wire under normal conditions of manufacture and severe conditions of use.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be better understood from the following specification when read in conjunction with the accompanying drawings in which:

FIG. 1 is a cross sectional view through the center of a preferred embodiment of a Geiger-Muller gas discharge radiation detecting device in accordance with this invention;

FIG. 2 is a cross sectional view thereof taken through line 22 of FIG. 1; and

FIG. 3 is a cross sectional view through an element thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to the drawings, a Geiger-Muller tube is illustrated in FIG. 1 and has a cylindrical, conducting metallic cathode ll surrounding a coaxially extending, conducting metallic anode wire 12 with an ionization chamber 13 therebetween closed by ceramic insulating pieces 14, 14'. Conductive metal end caps 15 and 16 have joints l7 and 18 frictionally holding the anode 12 in tension.

The cathode 11 is preferably formed of a stainless steel material having a high chromium content such as American Iron and Steel Institute (A.I.S.I.) type 446 stainless steel. As known in the art, the inner surface of the cylindrical cathode 11 is coated with a thin layer of platinum to improve the particle absorbing properties of the cathode. However, the platinum layer can be eliminated in some embodiments. Stainless steel electrode materials are commonly used in Geiger-Muller tubes because of their high strength and resistance to deterioration by the action of the gaseous atmosphere within the tube. The anode wire 12 is also formed of a stainless steel material since it is important that both the anode and cathode have substantially similar heat expansion characteristics in Geiger-Muller tubes designed for operation at elevated temperatures.

It has now been found that advantages are obtained when the anode wire is made of a high strength stainless steel such as A.I.S.I. l7-7PH stainless steel having a high chromium content. The higher strength of the anode material as compared with the cathode material enables greater tensioning than normally used in Geiger-Muller tube anodes of 446 stainless steel.

The chamber 13 between the anode and cathode can be filled with any of the conventionally used gaseous atmospheres. For example, the gas pressure within the chamber is normally between 0.1 to 0.4 atmospheres. The gas itself is normally a mixture of inert gases with "at least one quenching halogen gas. Gas compositions can be for example 98% neon, 1.5% argon with a halogen mixture of bromine and chlorine comprising the remaining 0.5%. The argon can vary to 98% with the neon varying to 1.5%. The proportions of bromine to chlorine can vary from 100% of either to 50-50 mixtures. The total amount of halogen gas can vary from 0.1 to 1 percent of the total gas atmosphere. The variations in the gas composition and the specific gases used produce different performance characteristics of the resulting Geiger-Muller tubes, i.e., starting voltage, plateau length and recovery time between counts, as known in the art.

Ceramic insulating pieces 14 and 14 are preferably identical. Each comprises a cylindrical body portion 20, an inwardly extending skirt 23 and a recessed cylindrical portion 21 to which an end cap such as 15 can be sealed. The specific shape of the ceramic end pieces can vary greatly although the shape shown is preferred to enhance ease of hermetic sealing by conventional glass frits 24 and 25.

The ceramic insulating pieces 14 and 14' are made of conventional ceramic materials preferably having thermal coefficients of expansion closely matching the thermal coefficients of expansion of the metals as for example 10.5 X l0' /C. Forsterite (2MgOSiO2) is a particularly useful material for the ceramic end pieces. The glass frits can be formed of conventional frit materials preferably having sealing temperatures varying from 450 to 550C and thermal coefficients of expansion closely matching those of the ceramic insulating pieces 14 and 14'.

The anode wire 12 is surface treated to provide resistance to attack from the gaseous atmosphere within the chamber 13. The surface treatment can be any of the known conventional surface treatments for anode wires used in conventional Geiger-Muller tubes. Such surface treatments typically provide an oxide coating on the anode over its entire surface exposed to the gas atmosphere. It has now been found that a thin coating of chromium can be plated on the anode preferably after conventional heat treatment, to provide an improved protective surface finish. Conventional vapor deposition methods can be used to coat the chromium layer with a thickness of from 100 to 2,000 angstroms. This coated layer is then heat treated to form chromium oxide at the surface thereby providing the desired protection against the gaseous atmosphere. The coating step is particularly useful when using high strength stainless steel such as l7-7PH since such steels often contain lower chromium contents than conventionally used 446 stainless steel.

An important feature of the invention is the mounting of the anode at both end portions by the end caps or filament pieces 15 and 16 through swaged friction joints illustrated at 17 and 18. The joints l7 and 18 are identical and only one will be fully described.

As clearly seen in FIG. 3, the anode 12 is held at the joint 18 by a friction fit along an elongated surface portion indicated at S. The joint is formed by bringing semicircular dies down on a tubular portion 37 of the end cap 16 completely about the circumference of the anode without substantially reducing the diameter of the anode. Preferably the diameter of the anode is uniformly reduced by less than 5 percent and preferably from 1 to 3 percent to thereby assure a satisfactory friction fit rather than a mechanical interlock by angled surfaces. If greater reduction is carried out, the anode is mechanically weakened so that high tensioning of the anode can result in unwanted stretching and permanent elongation. This friction fit joint is made at both ends of the anode allowing high tensioning and thus reducing flexing of the anode during conditions of use involving high vibration. The axial length of the swaged joint can be varied to obtain the desired holding force. However, in all cases, length S is always at least 3 times the length of the diameter of the wire anode. If the anode is to be joined to one end cap prior to sealing, in the device, one cap can have its tubular portion 37 pointed inwardly toward the chamber rather than outwardly as shown.

The end cap preferably has a radially outwardly extending flange 30 joined to an integral cylindrical skirt 31 which is mounted to the insulating end piece 14 by a glass frit. Gas fill holes 32 and 33 are preferably provided in the flange 30 of one of the end caps to allow introduction of the desired gas atmosphere in the chamber 13. An integral cylindrical tube portion 37 is preferably welded into a bead seal 34 although it can be sealed by a glass frit or the like. The dimensioning of each end piece provides for sufficient tolerance between the skirt 31 and flange 30 as at areas 35 and 36 so that the anode, even when flexed during usage, never touches these portions of the end cap thereby preventing damage to the surface coating. End cap 16 is preferably identical to end 'cap 15 except that holes 32 and 33 are eliminated or are welded closed prior to assembly of the device.

A tubular glass end protector 50 is preferably hermetically sealed to one ceramic insulating piece 14' by a conventional glass frit seal. The tubular protector is used to permit filling and flushing of the chamber during manufacture and is closed by forming an end sealed bead in a final manufacturing step.

In the manufacture of Geiger-Muller tubes in accordance with the invention, a stainless steel cathode tube 11 typically having a length of from 3 to 6 inches and a diameter of from one-fifth inch to 2 inches with a wall thickness of from 0.020 to 0.025 inch is cleaned and a thin platinum layer electroplated on its inner surface after which the inner surface is passivated by conventional steps. The cathode is then assembled with insulating end pieces 14, 14' and conductive metal end caps 15 and 16 in the position shown in FIG. l with glass frits used to form the seals shown as at temperatures of from 500 to 600C. The heat used in melting of the frits can be used to passivate the cathode layer if desired or it can be pre-passivated prior to assembly.

In the next step, the anode wire is inserted through the tubular portions of the end caps 15 and 16 before the ends 34 are closed. The anode wire is then tensioned and swaged by semicircular dies to form the mechanical friction joints shown. In some cases, the anode can be attached to one end cap and assembled with the cathode after which the second joint can be formed. The tube 50 is then sealed in position with its outer end opened.

If desired, the anode can be heat treated to form a surface coating of oxide before assembly although it is also possible to do it during assembly. For example, the anode can be heated after the forming of joints l7 and 18 to a temperature of from 800 to 900C in an air atmosphere for from 2 to 10 minutes. The anode can then be cooled to room temperature. After cooling to room temperature, when using l7-7PH stainless steel, it is preferred that no more than 2 hours lapse before carrying out the next step which is to cool the anode to a temperature between 5 0C to l 00C for at least 18 hours. The cooling step transforms the material from the austenite form to the stronger martensite form. Additional anode strength can be obtained in the wire by again heating after cooling as previously described to a temperature of from 400 to 600C for about an hour, which causes a solid state precipitation strengthening.

' In the next step, the gas mixture used is passed into the chamber 13 through holes 32 and 33, the assembly is heated to a temperature higher than its expected operating temperature, then evacuated and refilled with the desired gas mixture through holes 32 and 33. The tube 50 is then sealed at its outer end.

It should be understood that other heat treating methods known in the art can be used depending upon the specific materials used, dimensions of parts, expected operation temperature and desired detecting properties.

In all cases, the wire anode 112 is maintained in tension by joints l7 and 18 at a value of from at least 60,000 p.s.i. to no more than 67 percent of its yield point to obtain the benefits of this invention. In halogen quenched Geiger-Muller tubes of this invention the wire anode preferably has a uniform diameter in the range of from 0.009 to 0.040 inch. If lower tensioning is used, one obtains too much flexing which can destroy the coating on the anode while if higher tensioning is used, stretching of the wire occurs with poor fatigue life resulting. I

Preferably the wire anode is coated with a thin layer of a halogen resistant material which preferably uniformly covers the surface exposed to the halogen atmosphere. lt is known that halogen interacts with the iron based wire anodes used at high operating temperatures. This interaction can remove or reduce the small amount of free halogen present and lead to electrical breakdown. Known surface oxide layers help to alleviate this problem. However, substantially improved results are obtained by providing an adherent deposited coating of from to 2,000 angstroms of a halogen resistant protective coating over the wire. This coating can be deposited on the wire before conventional heat treatment but is preferably provided over the wire after formation of an oxide layer by conventional heat treatment. Coating can be of any material having a higher halogen resistant characteristic than that of the base. For example, when I7-7PH stainless steel is used for the wire anode the coating can be 446 stainless steel. However, the coating is preferably chromium. The coating is then oxidized at the exposed surface to provide the desired surface protection.

The coating can be formed by conventional steps including vapor deposition, sputtering and the like. For example, a thin chromium coating can be deposited on the oxidized surface of a l7-7PH stainless steel wire by passing vaporized chromium in a vacuum over the wire anode. The resultant coating is then oxidized by conventional means as by heating in air at from 350C to 700C for from l minutes to 2 hours or more to obtain a uniform complete oxide coating.

In a specific example of this invention, a 446 stainless steel cathode having a 0.001 inch platinum passivated inner surface is used with the cathode having a length of 4 inches, a diameter of 0.300 inch and a wall thickness of 0.025 inch. The anode wire has a length of 4.6 inches, a diameter of 0.025 inch. Tubular portions 37 of the end caps have a wall thickness of 0.020 inch and have a radial clearance of 0.002 inch from the anode wire. Length S is 0.l50 inch.

The cathode 11 is assembled with the insulating pieces 14 and 14, end caps 15 and 16, opened tubular end 50 and with the glass frits in position. The assembly is heated to 550C to melt the frits and form the hermetic seals.

The anode 12 is l7-7PH stainless steel having a uniform diameter of 0.025 inch. The anode is heated prior to assembly at 850C for 5 minutes and then cooled in air to room temperature. One hour after reaching room temperature it is put on dry ice and maintained at -75C for hours and then allowed to cool to room temperature.

The anode wire 12 is then positioned through tubular portion 37 and one swage joint formed by semicircular swaging dies forming a continuous friction joint completely around the diameter of the anode. The joint is formed with a length S of 0.150 inch and reduction of the wire diameter of 2 percent. The wire anode is then tensioned to 30 pounds and the second joint swaged as above described. Ends 34 are welded closed.

A further heating step is carried out at 450C for 1 hour. A gas mixture consisting of by volume 90% neon, 5% argon, 2.5% chlorine and 2.5% bromine is then passed into the chamber 13 and the assembly heated to 350C for 120 minutes after which the chamber is purged and refilled with a fresh gas mixture of 98% neon, 1.5% argon, 0.4% chlorine and 0.1% bromine at 0.2 atmosphere. The end of the glass tube 50 is hermetically sealed shut as shown in FIG. 1.

When the above device of the specific example is tested at an operational temperature of 135C and vibrated at resonant frequency of the anode (750 cycles per second) with peak acceleration of 20 GS, it gives reliable and accurate gamma ray counting results for periods at least as long as 700 hours. This compares drastically with known Geiger-Muller tubes such as one substantially as described but having one crimped joint end cap and one end cap removed and replaced by an interlocking conventional bead seal between the anode and the ceramic end piece 14. This device, undergoing the same test, is useless after 12 hour s use due to elec '7 trical breakdown of the anode.

In a second specific example the above example is repeated with one additional procedure. After the heat treatment of the wire anode and just prior to insertion in the assembled cathode, the wire anode is coated with a 1,000 angstrom thick uniform coating of chromium by vapor deposition. The coated anode is then heat treated in air at 600C for 30 minutes, allowed to cool and then assembled as previously described. This device gives reliable and accurate counting under the test conditions of the previous example for periods of at least 800 hours.

While specific examples of the invention have been described above, many variations are possible. For example, the specific friction sealing at both ends of an anode wire in a gas discharge radiation detecting tube is helpful to prolong tube life with otherwise wholly known tube structures. Similarly, coating of the central anode of conventional halogen quenched radiation de tecting tubes prolongs the life of such tubes under conditions of high vibration. The specific dimensions and configurations of parts can vary greatly depending upon the desired characteristics to be obtained and the operating conditions of the devices.

What is claimed is:

l. A radiation detecting device comprising,

a first tubular electrode,

a second elongated electrode spaced from and within said first tubular electrode,

insulating means defining in part a hermetically sealed gas ionization chamber between said first and second electrode, a pair of second electrode mounting means for mounting and tensioning said second electrode,

said mounting means being mechanically joined to said second electrode by friction joints at axially spaced portions of said second electrode.

2. A radiation detecting device in accordance with claim 1 wherein said second electrode is tensioned to at least 60,000 p.s.i. and no more than 67% of its yield point with said joints holding said electrode so that said electrode slides with respect to said joints under tension loads above a predetermined value and below the yield strength of the electrode.

3. A radiation detecting device in accordance with claim 2 wherein said first electrode is a cylinder and said second electrode is a coaxially extending uniform diameter wire.

4. A radiation detecting device in accordance with claim 3 wherein said first and second electrodes are formed of stainless steel and said second electrode has an oxidized surface.

5. A radiation detecting device in accordance with claim 4 wherein said wire is reduced in diameter at said joints by from 1 to 5%.

6. A radiation detecting device in accordance with claim 5 wherein said joints each have an axial length of at least three times the diameter of said wire.

7. A radiation detecting device in accordance with claim 6 wherein said gas ionization chamber contains a gas mixture comprising a halogen quenching gas.

8. A radiation detecting device in accordance with claim '7 wherein said second electrode is formed of l7-7 PH stainless steel having a diameter of from 0.009 to 0.040 inch.

9. A radiation detecting device in accordance with claim 8 wherein said first electrode is formed of 446 stainless steel.

10. A radiation detecting device in accordance with claim 9 wherein said second electrode carries an oxidized surface coating of chromium.

11. A radiation detecting device in accordance with claim 2 wherein said second electrode is an iron based alloy and carries a coating of a material which forms an oxide more resistant to chemical reaction with halogen than is the oxide of said alloy.

12. A radiation detecting device in accordance with claim 11 wherein said coating is chromium.

13. A radiation detecting device in accordance with claim 12 wherein said coating has a thickness of from to 2,000 Angstroms.

14. A radiation detecting device in accordance with claim 13 wherein said joints are each swaged joints.

15. The improvement of claim 14 wherein,

said elongated electrode is a high strength stainless steel wire having a uniform diameter and said joints are swaged to reduce the diameter of the wire by from 1 to at said joints with each of said joints having an axial length of at least 3 times the length of said diameter.

16. In a radiation detecting device having a first tubular electrode and a second elongated electrode coaxially extending within said tubular electrode, the improvement comprising,

end cap means defining in part a hermetically sealed chamber between said tubular electrode and said elongated electrode,

and means for maintaining said elongated electrode in tension within said chamber,

said means comprising a plurality of friction joints constructed and arranged to allow sliding of said elongated electrode with respect to said joints at a tension load less than the yield strength of said elongated electrode.

17. In a radiation detecting device for detecting charged particles and having a halogen gas containing hermetically sealed chamber, a first tubular electrode and a second elongated electrode spaced from and coaxially extending therein,

the improvement comprising,

said elongated electrode being formed of a high mechanical strength iron base alloy carrying a thin coating of material having high resistance to chemical attack by halogens,

said coating being formed of a high chromium containing material having an oxidized outer surface.

18. The improvement of claim 17 wherein said iron based alloy is a stainless steel having an oxide layer over which lies said coating of a chromium containing material.

119. The improvement of claim 18 wherein said material consists essentially of chromium having an oxidized outer surface and a thickness within the range of from to 2,000 Angstroms.

20. In a radiation detecting device having a first tubular electrode and a second elongated electrode coaxially extending within said tubular electrode with a hermetically sealed chamber lying in part between said first and second electrodes, the improvement compriss,

said elongated electrode being formed of a high mechanical strength iron base alloy carrying a thin coating of material having high resistance to chemical attack by halogens,

means comprising a sleeve surrounding a portion of said elongated electrode for maintaining said elongated electrode in tension,

said sleeve being frictionally joined to said elongated electrode and arranged to allow sliding of said elongated electrode with respect to said sleeve at tension loads less than the yield strength of said elongated electrode.

21. The improvement of claim 20 wherein, said elongated electrode is a high strength stainless steel wire having a uniform diameter and said joint is swaged to reduce the sleeve and reduce the diameter of the wire by from 1 to 5% at said joint with said joint having an axial swaged length of at least 3 times the length of said diameter. 

1. A radiation detecting device comprising, a first tubular electrode, a second elongated electrode spaced from and within said first tubular electrode, insulating means defining in part a hermetically sealed gas ionization chamber between said first and second electrode, a pair of second electrode mounting means for mounting and tensioning said second electrode, said mounting means being mechanically joined to said second electrode by friction joints at axially spaced portions of said second electrode.
 2. A radiation detecting device in accordance with claim 1 wherein said second electrode is tensioned to at least 60,000 p.s.i. and no more than 67% of its yield point with said joints holding said electrode so that said electrode slides with respect to said joints under tension loads above a predetermined value and below the yield strength of the electrode.
 3. A radiation detecting device in accordance with claim 2 wherein said first electrode is a cylinder and said second electrode is a coaxially extending uniform diameter wire.
 4. A radiation detecting device in accordance with claim 3 wherein said first and second electrodes are formed of stainless steel and said second electrode has an oxidized surface.
 5. A radiation detecting device in accordance with claim 4 wherein said wire is reduced in diameter at said joints by from 1 to 5%.
 6. A radiation detecting device in accordance with claim 5 wherEin said joints each have an axial length of at least three times the diameter of said wire.
 7. A radiation detecting device in accordance with claim 6 wherein said gas ionization chamber contains a gas mixture comprising a halogen quenching gas.
 8. A radiation detecting device in accordance with claim 7 wherein said second electrode is formed of 17-7 PH stainless steel having a diameter of from 0.009 to 0.040 inch.
 9. A radiation detecting device in accordance with claim 8 wherein said first electrode is formed of 446 stainless steel.
 10. A radiation detecting device in accordance with claim 9 wherein said second electrode carries an oxidized surface coating of chromium.
 11. A radiation detecting device in accordance with claim 2 wherein said second electrode is an iron based alloy and carries a coating of a material which forms an oxide more resistant to chemical reaction with halogen than is the oxide of said alloy.
 12. A radiation detecting device in accordance with claim 11 wherein said coating is chromium.
 13. A radiation detecting device in accordance with claim 12 wherein said coating has a thickness of from 100 to 2,000 Angstroms.
 14. A radiation detecting device in accordance with claim 13 wherein said joints are each swaged joints.
 15. The improvement of claim 14 wherein, said elongated electrode is a high strength stainless steel wire having a uniform diameter and said joints are swaged to reduce the diameter of the wire by from 1 to 5% at said joints with each of said joints having an axial length of at least 3 times the length of said diameter.
 16. In a radiation detecting device having a first tubular electrode and a second elongated electrode coaxially extending within said tubular electrode, the improvement comprising, end cap means defining in part a hermetically sealed chamber between said tubular electrode and said elongated electrode, and means for maintaining said elongated electrode in tension within said chamber, said means comprising a plurality of friction joints constructed and arranged to allow sliding of said elongated electrode with respect to said joints at a tension load less than the yield strength of said elongated electrode.
 17. In a radiation detecting device for detecting charged particles and having a halogen gas containing hermetically sealed chamber, a first tubular electrode and a second elongated electrode spaced from and coaxially extending therein, the improvement comprising, said elongated electrode being formed of a high mechanical strength iron base alloy carrying a thin coating of material having high resistance to chemical attack by halogens, said coating being formed of a high chromium containing material having an oxidized outer surface.
 18. The improvement of claim 17 wherein said iron based alloy is a stainless steel having an oxide layer over which lies said coating of a chromium containing material.
 19. The improvement of claim 18 wherein said material consists essentially of chromium having an oxidized outer surface and a thickness within the range of from 100 to 2,000 Angstroms.
 20. In a radiation detecting device having a first tubular electrode and a second elongated electrode coaxially extending within said tubular electrode with a hermetically sealed chamber lying in part between said first and second electrodes, the improvement comprising, said elongated electrode being formed of a high mechanical strength iron base alloy carrying a thin coating of material having high resistance to chemical attack by halogens, means comprising a sleeve surrounding a portion of said elongated electrode for maintaining said elongated electrode in tension, said sleeve being frictionally joined to said elongated electrode and arranged to allow sliding of said elongated electrode with respect to said sleeve at tension loads less than the yield strength of said elongatEd electrode.
 21. The improvement of claim 20 wherein, said elongated electrode is a high strength stainless steel wire having a uniform diameter and said joint is swaged to reduce the sleeve and reduce the diameter of the wire by from 1 to 5% at said joint with said joint having an axial swaged length of at least 3 times the length of said diameter. 